1. Technical Field
This disclosure relates to a method and a device for implementing hierarchical modulation of data streams.
2. Description of Related Art
Hierarchical modulation, also called layered modulation, is a signal processing technique for multiplexing and modulating multiple data streams into one single symbol stream. The idea is that one stream (i.e., the base-layer, also called the High Priority stream, HP) is modulated with a robust modulation technique and one or more other streams (i.e., enhancement-layers, also called Low Priority streams, LP) are synchronously superimposed on the base layer with a less robust modulation. When hierarchical modulation signals are transmitted, user devices with good reception and with hierarchical modulation enabled receivers can demodulate the base and enhancement layers, while user devices with poor reception or conventional receivers can demodulate only the base layer (the HP stream). Hierarchical modulation has been used in, among other things, video broadcasting such as digital video broadcasting—terrestrial (DVB-T), digital video broadcasting—handheld (DVB-H), and MediaFLO by Qualcomm Incorporated, the implementation of which are readily apparent to one of ordinary skill in the art.
FIG. 1 illustrates a conventional video broadcasting hierarchical modulation system 100. Broadcasting refers to the transmission of a message to all receivers (e.g., every device on the network) simultaneously. Here, the base layer is associated with a video base layer that is needed for all receivers (e.g., user equipment UE1 120 and UE2 130, although any number of UEs may be implemented), which is sent from a base station 110 with high robustness to guarantee proper decoding by all receivers even with poor channel quality. Enhanced layers, which only contribute to the quality of the received video, are sent with less robust techniques. Hence, receiving devices with good channel conditions, e.g., UE1, can receive the full video signal, while those devices with poor channel conditions at the edge of a cell, e.g., UE2 , will only manage to get the base layer. A UE may also be referred to as a user device herein.
FIG. 2A illustrates a constellation diagram of a basic implementation of hierarchical modulation. Here, the base layer is viewed as quadrature phase shift keying (QPSK also referred to as quadrature amplitude modulation QAM4) (shown left) with two bits per symbol data carrying capacity, while the enhanced layer is provided by the additional two bits per symbol data carrying capacity (4 bits per symbol total) of quadrature amplitude modulation QAM16 (shown right with base layer bits and enhanced layer bits separated by the “|” character for clarity). Receiving the base layer includes only determining which quadrant the symbol resides in, and can be performed by demodulating the received signal as if it were transmitted as QPSK. Receiving the enhanced layer includes demodulating the received signal as QAM16 to determine the position within the quadrant to resolve the additional two bits of data carrying capacity.
FIG. 2B shows an example of hierarchical modulation in which the base layer and enhanced layer are mapped into QAM64, which logically appears to be a QAM16 (represented by the 16 square constellation points in each quadrant) constellation with 4 bits per symbol data carrying capacity on top of a QPSK (represented by the gray circles) constellation with 2 bits per symbol data carrying capacity realized by a single transmission of a QAM64 constellation with 6 bits per symbol data carrying capacity. In this case, to receive the enhanced layer it might be useful to receive the signal as QAM64, but to receive the base layer it is only useful to receive the signal as QPSK. One skilled in the art would understand that different variants, such as receiving the base layer as QAM16 and the enhanced layer as the remaining 2 bits per symbol carrying capacity of a QAM64 constellation, are possible.
While traditional hierarchical modulation has been used in broadcasting, it has not been used in non-broadcasting communications system such as, but not limited to unicast transmission. Unicast transmission refers to the sending of messages to a single network destination. It would be helpful to find a way to use hierarchical modulation in a non-broadcast communications system to better utilize bandwidth capacity.